Article pubs.acs.org/JACS
Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction Dong Young Chung,†,‡,# Samuel Woojoo Jun,†,‡,# Gabin Yoon,†,§ Soon Gu Kwon,†,‡ Dong Yun Shin,∥ Pilseon Seo,†,‡ Ji Mun Yoo,†,‡ Heejong Shin,†,‡ Young-Hoon Chung,⊥ Hyunjoong Kim,†,‡ Bongjin Simon Mun,∇ Kug-Seung Lee,Δ Nam-Suk Lee,○ Sung Jong Yoo,⊥ Dong-Hee Lim,∥ Kisuk Kang,†,§ Yung-Eun Sung,*,†,‡ and Taeghwan Hyeon*,†,‡ †
Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, South Korea School of Chemical and Biological Engineering & §Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, South Korea ∥ Department of Environmental Engineering, Chungbuk National University, Chungbuk 361-763, South Korea ⊥ Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea ∇ Department of Physics & Photon Science, Ertl Center for Electrochemistry and Catalyst, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea Δ Pohang Accelerator Laboratory (PAL) & ○National Institute for Nanomaterials Technology (NINT), Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea ‡
S Supporting Information *
ABSTRACT: Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a “dual purpose” N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the Ndoped carbon shell that is in situ formed from dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, we achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, we accomplished the long-term stability in membrane electrode assembly (MEA) for 100 h without significant activity loss. From in situ XANES, EDS, and first-principles calculations, we confirmed that an ordered fctPtFe structure is critical for the long-term stability of our nanocatalyst. This strategy utilizing an N-doped carbon shell for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting the catalyst during fuel cell cycling is expected to open a new simple and effective route for the commercialization of fuel cells.
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decrease with their sizes. 16 In these regards, ordered intermetallic NPs are considered as one of the most promising candidates to achieve both high activity and stability in practical fuel cell applications.17−23 The Sun group reported that the ordered intermetallic PtFe NPs of face-centered tetragonal (fct) structure have superior iron antidissolution property under the acidic fuel cell operating condition. In addition, fct-PtFe NPs show higher activity than the disordered face-centered cubic (fcc) PtFe NPs.17 The Abruna group reported the superior long-term stability of ordered fct-PtCo NPs,18 which is attributed to the Pt-rich surface and the strong Pt−Co bonding
INTRODUCTION Nanoparticle-based electrocatalysts have been intensively investigated for fuel cell applications over the past decade, mainly motivated by their high mass activity.1−14 Many research groups have made great effort to utilize the high activity and surface area of nanoparticles (NPs) in order to make a breakthrough for fuel cell commercialization. However, practical use of nanomaterials for fuel cell electrocatalyst was impeded by their low physical and chemical stability. Under the standard fuel cell operating conditions, NPs are often oxidized, dissolved, or detached from the support and agglomerated into larger particles, losing their electrochemical catalytic activity during cycling.15 Moreover, according to theoretical calculations, the oxidation and dissolution potentials of NPs tend to © 2015 American Chemical Society
Received: September 14, 2015 Published: December 3, 2015 15478
DOI: 10.1021/jacs.5b09653 J. Am. Chem. Soc. 2015, 137, 15478−15485
Article
Journal of the American Chemical Society
Figure 1. Synthesis of ordered intermetallic fct PtFe/C. (a) Schematic synthesis diagram of carbon-supported and N-doped carbon-coated ordered fct-PtFe NPs. (b-c) TEM images of thermally annealed NPs without (b) and with dopamine coating (c) at 700 °C. (d) HRPD data of carbonsupported pure Pt (Pt/C), PtFe NPs before annealing (PtFe/C, before), and after annealing (PtFe/C, after). (e-g) Cs-corrected HAADF-STEM image of a fct-PtFe NP after annealing (e), a model structure of fct-PtFe structure (f), and a FFT patterns of the image (g). The Pt columns look brighter than the Fe columns due to the Z-contrast.
agglomerate during the fuel cell operation.32 However, it was criticized that, although with enhanced stability, the presence of a protective layer can lower the catalytic performance by blocking the active sites and lowering the mass transport rate at the surface of the NPs. Being inspired by those two different approaches to improve the long-term stability of nanocatalysts, we designed a nanoparticle-based electrocatalyst that combines those approaches together and overcomes their limitations. Herein, we report a synthetic method of highly durable and active PtFe NPs with a very thin “dual purpose” N-doped carbon shell. This carbon shell is in situ formed from a polydopamine coating of disordered fcc-PtFe NPs during thermal annealing to transform the NPs into an ordered fct structure. The carbon shell effectively prevents PtFe NPs from coalescence during the annealing so that fct-PtFe NPs as small as 6.5 nm can be obtained. Moreover, by controlling the thickness of the carbon shell at subnanometer scale, the shell also acts as a highly effective protective coating of the NPs during fuel cell cycling while it is still permeable for the reactant molecules and not affecting their electrocatalytic activity. In half-cell and membrane electrode assembly (MEA) tests, our N-doped carbon-coated PtFe NPs show excellent long-term stability for realistic fuel cell application. The origin of the stability of our nanocatalyst is investigated with in situ synchrotron X-ray absorption spectroscopy and first-principles calculation model-
in the core of the NPs. Despite these advantages, however, the synthesis of small-sized ordered fct-PtFe NPs with high mass activity has been rarely reported thus far. In general, assynthesized PtFe NPs have disordered fcc structure, and consequently thermal annealing at ∼700 °C is necessary to transform them into an ordered fct structure,24 which inevitably leads to the coalescence of the NPs forming larger and polydisperse NPs. To prevent this coalescence during annealing, protective coating of the NPs with inorganic shells17,25 or physical barriers26 has been suggested. However, this approach requires an additional step of removing the coating layer from the surface of the NPs to expose the active sites, which increases time and cost in the production. In the meantime, there has been another approach to utilize protective coating to enhance the long-term performance of fuel cells. By coating NPs deposited on the support with a protective layer, they can be made more resistive toward detachment and agglomeration, which are the major deactivation mechanisms during fuel cell operation, as mentioned above.27So far, various coating layers, including a carbon shell,28 an inorganic barrier,29 and graphitic hollow spheres,30,31 are proposed, and the protective layer-coated NPs show better performance in terms of the long-term stability compared to the uncoated counterparts. Especially, it is known that N-doped carbon has very good affinity to the surface of metal NPs and, thus, stabilizes the small NPs not to 15479
DOI: 10.1021/jacs.5b09653 J. Am. Chem. Soc. 2015, 137, 15478−15485
Article
Journal of the American Chemical Society
Figure 2. Characterizations of N-doped carbon shells. (a) TEM images of carbon-loaded and N-doped carbon shell-coated PtFe NPs and plots of the shell thickness (number of carbon interlayers) vs polydopamine coating time. (b) HR-PES data around the N 1s core level from PtFe NPs before annealing (top), after annealing (middle), and without dopamine coating (bottom). (c) HR-TEM image of a carbon-supported and carbon shellcoated PtFe NP (left) and its EF-TEM image showing the distribution of nitrogen (right).
The characteristic superlattice peaks of the ordered intermetallic fct structure appear after the annealing, as marked with asterisks. Also, high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and fast Fourier transform (FFT) analysis (Figures 1a−1g) confirm the ordered fct structure of PtFe NPs. In these images, the unit cell projected along the [001] axis shows a periodic line array of Fe columns stacked by Pt columns. Polydopamine is used as the precursor for in situ formation of an N-doped carbon shell.34 The thickness of the N-doped carbon shell can be precisely controlled by the coating time of polydopamine.35 TEM data show that the thickness of the polydopamine coating is roughly proportional to the coating time (Figure 2a). In the case of 1 h coating, the carbon shell with average thickness of ∼0.43 nm is formed, which amounts to approximately two layers of carbon sheet (Figure S3). In high resolution photoemission spectroscopy (HR-PES) data, the presence of N−C and N−H bonding is confirmed in the N 1s signal from polydopamine-coated NPs before the annealing (Figure 2b). After thermal annealing at 700 °C, carbonization of dopamine takes place and signals from pyridinic (N 1s ∼ 399 eV), pyrrolic (N 1s ∼ 400 eV), and graphitic (N 1s ∼ 401 eV) nitrogen are observed, indicating that dopamine is transformed into an N-doped carbon shell. From a controlled sample without dopamine coating, no nitrogen signal is detected. The ratio of N 1s and Pt 4f signal intensities, which is proportional to the thickness of the dopamine coating of PtFe NPs, is increased with the coating time (Figure S4), further supporting the result from the TEM data. Bright field and energy-filtered TEM (EF-TEM) images in Figure 2c and Figure S5 reveal that
ing, and the results confirm the critical importance of the ordered fct structure and the presence of an N-doped carbon shell for the long-term stability.
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RESULTS AND DISCUSSION Preparation of carbon shell-coated PtFe nanocatalyst. The formation process of ordered fct-PtFe NPs with a dual purpose N-doped carbon shell is illustrated in Figure 1a. First, disordered fcc-PtFe NPs are synthesized via the previously reported method using platinum acetylacetonate [Pt(acac)2] and iron pentacarbonyl [Fe(CO)5] as Pt and Fe precursors, respectively (Supporting Information, Figure S1).33 Assynthesized PtFe NPs are supported on carbon particles and then coated with polydopamine by a treatment with dopamine hydrochloride solution. Thermal annealing of carbon-supported and dopamine-coated fcc-PtFe NPs at 700 °C leads to the formation of atomically ordered fct-PtFe NPs with an N-doped carbon coating. The effect of an in situ formed N-doped carbon shell is investigated by performing a control experiment without a dopamine coating. As shown in Figures 1b and 1c, thermal annealing of carbon-supported PtFe NPs (PtFe/C) without a dopamine coating leads to a significant increase in the NP size up to tens of nanometers due to extensive coalescence. On the other hand, 6.5-nm-sized NPs are obtained from thermal annealing of dopamine-coated PtFe/C, which is close to the initial size of the NPs before the annealing (Figure S2), which confirms that the in situ formed carbon shell makes the NPs highly resistive toward coalescence. The ordered intermetallic fct structure of PtFe/C after the annealing is confirmed by high resolution powder diffraction (HRPD) in Figure 1d in comparison with PtFe/C before annealing and pure Pt/C. 15480
DOI: 10.1021/jacs.5b09653 J. Am. Chem. Soc. 2015, 137, 15478−15485
Article
Journal of the American Chemical Society
Figure 3. Electrochemical oxygen reduction reaction activity. (a) Oxygen reduction reaction activity of ordered fct-PtFe/C, disordered fcc-PtFe/C, and Pt/C measured using a 1600 rpm rotating disc electrode. (b) Mass and specific activities of the electrodes measured at 0.9 V. (c and d) ORR activity of ordered fct-PtFe nanocatalysts modified with calix[4]arene thiol derivative with thin (c,
